The temperature in the combustion system of a gas turbine, e.g. in a gas turbine combustion chamber, has hitherto generally been determined by measuring the temperature at the turbine's exhaust outlet. From the measured turbine outlet temperature, the temperature in the combustion chamber is calculated allowing for the expansion and cooling undergone by the hot combustion gases in the turbine. The temperature in the combustion chamber has hitherto not been measured directly, as the temperature sensors used cannot be operated in the temperature range obtaining in the combustion chamber.
However, the temperature in the combustion chamber is a variable which is used to control the amount of fuel supplied to the combustion process. Whereas measuring the temperature at the turbine outlet allows the temperature in the combustion chamber to be deduced sufficiently well for controlling single-stage combustion, in the case of two-stage sequential combustion, as described e.g. in U.S. Pat. No. 6,050,078, it is difficult or even quite impossible to determine the temperature in the upstream combustion chamber with sufficient accuracy. Therefore, the problem of controlling two-stage sequential combustion on the basis of the turbine outlet temperature has hitherto not been satisfactorily resolved. This is the case particularly when a turbine stage is disposed between the upstream combustion chamber and the downstream combustion chamber, as described in U.S. Pat. No. 5,577,378, for example.